Optical safety element and system for visualising hidden information

ABSTRACT

The invention concerns an optical security element and a system for visualising items of concealed information comprising such an optical security element. The optical security element has a substrate layer in which a relief structure defined by relief parameters, in particular relief shape, relief depth, spatial frequency and azimuth angle, is shaped in the surface region defined by an X-axis and a Y-axis, for producing an optically perceptible effect. One or more of the relief parameters defining the relief structure are varied in the surface region in accordance with a parameter variation function. The surface region is divided into one or more pattern regions and a background region. One or more of the relief parameters defining the relief structure are varied in the one or more pattern regions in accordance with a parameter variation function which is phase-displaced in relation to the parameter variation function of the background region. There is further provided a verification element which has a verification grating which is defined by a periodic transmission function and whose period corresponds to the period of the parameter variation function.

The invention relates to an optical security element having a substratelayer in which a relief structure defined by relief parameters, inparticular relief shape, relief depth, spatial frequency and azimuthangle, is shaped out in a surface region defined by an X-axis and aY-axis, for producing an optically perceptible effect, and a system forvisualising items of concealed information with such an optical securityelement.

The ever improving photocopying technology and the ongoing developmentof electronic scanning and printing apparatuses mean that there is anincreasing need for optical security elements which are as forgery-proofas possible.

Now, U.S. Pat. No. 6,351,537 B1 describes an optical security elementwhich combines a hologram and a concealed image to increase the level ofcopying security. The hologram used is a daylight hologram which isgenerated by an optical-diffraction structure shaped in a photopolymerfilm and is visible without the use of a monochromatic, coherent lightsource. The concealed image and the hologram are preferably arranged inadjacent relationship on a substrate. The concealed image is renderedvisible by means of a decoding device. In that respect the decodingdevice used can be digital copiers or scanners but also transparentcarriers on which a line grating with a line spacing corresponding tothe desired scanning frequency is printed. In that case the concealedimage is produced from a starting image by a procedure whereby firstlythe frequency components of the starting image, which are greater thanhalf the scanning frequency of the decoding device, are removed and theremaining frequency components are then mirrored at the frequency axiswhich corresponds to half the scanning frequency.

In that way the optical security element affords a first securityfeature, namely the hologram, and a second security feature, namely theconcealed image. That enhances the level of safeguard against forgery.

U.S. Pat. No. 5,999,280 describes a holographic process for enhancingthe level of safeguard against forgery, in which a concealed image whichcan be perceived only by means of a special decoding device is shaped ina hologram. When the decoding device is moved over the hologram theconcealed pattern can then be visually detected by the viewer.

In that case such a hologram is generated in an encoding process from abackground image and from the image to be concealed in the hologram. Thebackground image comprises a line grating with a plurality of parallelblack stripes. Now, in the encoding process, those parts of the image tobe concealed, which are over the black stripes of the background image,are converted into white. Those parts of the image to be concealed,which are over the white part of the background image, are left black.Conversion into a hologram is effected by means of classic holographictechnologies, in respect of which there are limitations in respect ofthe grating structures which can be produced, by virtue of theunderlying physical principle.

Here however there is the disadvantage that such a security element canbe imitated by the use of holographic procedures.

Now the object of the invention is to improve the level of safeguardagainst forgery of optical security elements and to provide a system forthe visualisation of items of concealed information, which ensures ahigh degree of safeguard against forgery.

That object is attained by an optical security element in which a reliefstructure defined by relief parameters, in particular relief shape,relief depth, spatial frequency and azimuth angle is shaped in a surfaceregion of a substrate layer, which region is defined by an X-axis and aY-axis, for producing an optically perceptible effect, in which one ormore of the relief parameters defining the relief structure in thesurface region are varied in accordance with a parameter variationfunction, in which the surface region is divided into one or morepattern regions and a background region, and in which one or more of therelief parameters defining the relief structure in the one or morepattern regions are varied in accordance with a parameter variationfunction which is phase-displaced in relation to the parameter variationfunction of the background region. The invention is further attained bya system for visualising items of concealed information with such anoptical security element which further has a verification element with averification grating which is defined by a periodic transmissionfunction and whose period corresponds to the period of the parametervariation function.

The invention achieves a number of advantages: on the one hand it is notpossible to generate the relief structures necessary for the inventionby means of conventional holographic methods. That applies equally forthe optical effects generated by an optical security element accordingto the invention. They too cannot be imitated by means of conventionalholographic methods. Accordingly imitation by means of conventionalholographic methods is not possible. In addition novel optical effectsare generated if an optical security element according to the inventionis viewed through a verification element or a verification element ismoved over an optical security element according to the invention. Thusstriking colour and brightness changes occur in the movement and/or whenviewing from different viewing angles. Those novel visual effects areinherent in the relief structure of an optical security elementaccording to the invention so that imitation by other relief structureswhich are easier to produce are not possible. Accordingly an opticalsecurity element according to the invention provides a security featurewhich is very difficult to copy or to imitate but which on the otherhand can be easily verified by a user by means of an associatedverification element.

Advantageous configurations are set forth in the appendant claims.

In accordance with a preferred embodiment of the invention the reliefstructure in this case is formed by a diffraction grating whose azimuthangle is varied periodically in accordance with the parameter variationfunction in the surface region. If a verification element is applied toa surface region with such a relief structure, different optical effectscan be observed by the viewer on the one hand in dependence on thealignment and orientation of the verification element and on the otherhand in dependence on the viewing direction. For example a viewerperceives a surface region provided with such a relief structure,without using a verification element, from all viewing directions, as ahomogeneous surface region. In a first alignment of the verificationelement the pattern region and the background region appear in differentlevels of brightness, depending on the respective viewing direction. Ina second alignment or when viewing from a different viewing direction,the complementary effect occurs.

Accordingly a security feature which is easy to see but very difficultto imitate is generated in the surface region by such a reliefstructure.

The parameter variation function can in that case vary the azimuth angleof the diffraction grating periodically in dependence on the value ofthe X-axis. It is particularly advantageous if in that case theparameter variation function varies the azimuth angle of the diffractiongrating in such a way that the diffraction grating is composed of aplurality of lines in serpentine line form. Attractive optical effectswhich can serve as an additional security feature are produced by theuse of parameter variation functions of that kind, upon rotation of theverification element on the optical security element. In order toachieve such effects it is for example appropriate for the parametervariation function used to be a sine function which varies the azimuthangle of the diffraction grating in dependence on the value of theX-axis.

Still more complex security features which are therefore still moredifficult to imitate can be achieved if the parameter variation functionvaries the azimuth angle of the diffraction grating periodically independence on the value of the X-axis and in dependence on the value ofthe Y-axis. It is possible in that way to achieve further advantages inregard to the safeguard against forgery of the optical security elementaccording to the invention.

The parameter variation function can accordingly vary relief parametersin dependence on the value of the X-axis, in dependence on the value ofthe Y-axis, as well as in dependence on the value of the X-axis and independence on the value of the Y-axis.

The above-described diffraction grating whose azimuth angle is variedperiodically in accordance with the parameter variation functiondesirably is of a spatial frequency of more than 300 lines permillimetre, in particular 800 to 1200 lines per millimetre, so thatclearly perceptible difference in lightness come to light. It is furtheradvantageous for the parameter variation function to be so selected thatthe mean azimuth angle, in relation to the resolution capacity of thehuman eye, is constant in the surface region. That achieves ahomogeneous appearance in the surface region as long as no verificationelement is applied to the surface region.

In accordance with a further preferred embodiment of the invention therelief structure is a diffraction grating whose spatial frequency isvaried periodically in accordance with the parameter variation function.In that way it is possible for the surface region to exhibit differentcolour phenomena and colour changes in the pattern region and in thebackground region if a verification element is applied. Those differentcolour phenomena and colour changes can be easily seen by the viewer andcan therefore be particularly well used as a security feature.

Particularly clearly perceptible effects can be achieved when using aparameter variation function in which the spatial frequency of thediffraction grating is varied in dependence on the value of the X-axisperiodically between a maximum frequency, preferably 1200 lines permillimetre, and a minimum frequency, preferably 800 lines permillimetre. In that respect preferably sawtooth, triangular or sinefunctions are used as the parameter variation functions.

It will be appreciated that in this case also it is possible to useparameter variation functions which vary the spatial frequency not onlyin dependence on the X-axis but also in dependence on the Y-axis.Security features which are still more difficult to imitate can beachieved by such complex relief structures.

It is further advantageous in this case also for the parameter variationfunction to be constant in relation to the resolution capacity of thehuman eye in that way and thus for the surface region to afford ahomogeneous colour impression for a human viewer without the use of averification element.

In accordance with further preferred embodiments of the invention theperiodic parameter variation function varies the profile of the reliefstructure, thus for example varies the profile depth, the width of thedepressions or the profile shape. The use of such parameter variationfunctions makes it possible to produce security features which exhibitchanges in colour or lightness of the pattern region or of thebackground region when using a verification element. If the parametervariation function varies the profile shape periodically betweenasymmetrical, preferably mutually mirror-symmetrical, relief shapes,different effects, which are dependent on the viewing angle, appear inthe background region and in the pattern region, when using averification element, in dependence on the alignment of the verificationelement. In that way security features which are easy to see and verydifficult to imitate can also be generated in the surface region by thevariation in such parameters. In addition it is also possible for therelief structure to be a matt structure whose relief parameter, forexample scatter angle or preferred scatter direction (in the case ofanisotropic matt structures) is varied in accordance with the parametervariation function. Furthermore it is also possible for the parametervariation function to vary periodically between different kinds ofrelief structures, for example between a matt structure and adiffraction grating or a macrostructure.

In accordance with a further embodiment of the invention it is alsopossible for the relief structure to be a macrostructure with a spatialfrequency of fewer than 300 lines per millimetre. Thus for example thelight is reflected in different directions in dependence on the positionof the verification element in the pattern and background regions sothat a surface which is homogeneous without using a verification elementexhibits differences in lightness which are dependent on the viewingangle in respect of the pattern and background regions when using averification element.

It will be appreciated that it is also possible for the above-describedpossible options in respect of the variation in relief parameters by theparameter variation function to be combined together and thus forexample it is possible to vary both the azimuth angle and also thespatial frequency periodically by means of the periodic parametervariation function. Thus for example colour-dependent,lightness-dependent and viewing angle-dependent components can becombined to afford particularly impressive security features.

In the above-depicted embodiments of the invention it has proven to beparticularly advantageous for the period of the parameter variationfunction to be kept less than 300 μm and in particular for it to betaken from the range of 20 to 200 μm. That ensures that the patternregion cannot be distinguished from the background region by the humanviewer, without using the verification element.

Further advantages are afforded if the parameter variation function is afunction which is dependent both on the X-axis and also on the Y-axisand which is periodic in more than one direction, and in additiondifferent pattern regions are phase-displaced in relation to differentperiodicities. In that way it is possible to achieve motion effects whenrotating the verification element on the optical security elementaccording to the invention.

In the simplest case a simple line grating with a period correspondingto the periodicity of the parameter variation function is used as theverification element. In order further to enhance the safeguard againstforgery of a system according to the invention for visualisation ofitems of concealed information, it is in this case also possible to usea more complex line grating which for example comprises a plurality oflines in serpentine line form or a two-dimensional random pattern. Inthat case it is then also necessary for the mean variation of reliefparameters, which is produced by the parameter variation function, to beadapted to the surface pattern of that more complex line grating.

Further improvements in the degree of safeguard against forgery can beachieved if, instead of a binary verification grating, a verificationgrating is used which is defined by a non-binary transmission function,for example by a sinusoidal transmission function. Visualisation of theconcealed information accordingly requires a complex, individualisableverification element, whereby the level of forgery safeguard of thesystem is increased.

The invention is described by way of example hereinafter by means of anumber of embodiments with reference to the accompanying drawings inwhich:

FIG. 1 shows a diagrammatic view in cross-section through an opticalsecurity element according to the invention,

FIG. 2 a shows a functional diagrammatic view illustrating a portion ofa surface region of an optical security element according to theinvention as set forth in claim 1,

FIG. 2 b shows a surface region of the optical security elementaccording to the invention as shown in FIG. 1,

FIG. 2 c shows a view to illustrate the mode of operation in principleof the optical security element according to the invention as shown inFIG. 1,

FIG. 3 shows a diagrammatic view of a surface region of an opticalsecurity element according to the invention for a further embodiment ofthe invention,

FIG. 4 shows a diagrammatic view of a surface region of an opticalsecurity element according to the invention for a further embodiment ofthe invention,

FIGS. 5 a to 5 c show views of possible parameter variation functionsfor a further embodiment of an optical security element according to theinvention,

FIG. 5 d shows a view of a surface region of an optical security elementaccording to the invention for a further embodiment of the invention,

FIGS. 6 a and 6 b show a relief structure and a surface regionrespectively of an optical security element according to the inventionfor a further embodiment of the invention,

FIGS. 7 a to 7 e show surface regions and relief structures respectivelyof an optical security element according to the invention for a furtherembodiment of the invention,

FIGS. 8 a to 8 e show a surface region, a portion of a parametervariation function and a plurality of relief shapes of an opticalsecurity element for a further embodiment of the invention,

FIGS. 9 a and 9 b show diagrammatic views of relief structures of anoptical security element for a further embodiment of the invention,

FIGS. 10 a to 10 f show diagrammatic views of various verificationelements for a system according to the invention for the visualisationof items of concealed information, and

FIG. 11 shows a functional diagrammatic view of a system according tothe invention for the visualisation of items of concealed information.

FIG. 1 shows a stamping film 1 which has a carrier film 11 and atransfer layer portion 12 serving as an optical security element. Thetransfer layer portion 12 has a release and/or protective lacquer layer13, a replication layer 14, a reflection layer 15 and an adhesive layer16. The carrier layer 21 comprises for example a polyester film of athickness of 12 μm to 50 μm. Applied to the carrier film is the releaseand/or protective lacquer layer 13, in a thickness of 0.3 to 1.2 μm, aswell as the replication layer 14. It would also be possible in thatrespect to dispense with the release and/or protective lacquer layer 13.

The replication layer 14 is preferably a transparent thermoplasticmaterial which is applied for example by means of a printing process tothe film body formed by the carrier film 11 and the protective lacquerand/or release layer 13. After drying a relief structure 17 isreplicated into the replication layer in the region 18 by means of astamping tool. It is however also possible for the replication operationto be performed by means of a UV replication process, in which a UVreplication lacquer is applied to the film body formed by the carrierfilm 11 and the release and/or protective lacquer layer 13 and thenpartially irradiated with UV light for replication of the reliefstructure 17. After replication of the relief structure 17 in thereplication layer 14 the replication lacquer hardens by cross-linking orin some other fashion.

A thin reflection layer 15 is now applied to the replication layer 14.The reflection layer 15 is preferably a thin; vapour-deposited metallayer or an HRI layer (HRI=High Refraction Index). For example TiO₂, ZnSor Nb₂O₅ can be used as materials for an HRI layer. Essentiallychromium, aluminium, copper, iron, nickel, silver, gold or an alloy withthose materials can be used as the material for the metal layer. Inaddition instead of such a metallic or dielectric reflection layer, itis possible to use a thin film layer sequence with a plurality ofdielectric or dielectric and metallic layers.

The adhesive layer 16 which for example comprises a thermallyactivatable adhesive is now applied to the film body formed in that way.

To apply the optical security element to a security document or someother article to be safeguarded, the stamping film is applied with thetransfer layer portion 12 leading to the security document or to thearticle to be safeguarded and in that operation the carrier film 11 ispulled off the transfer layer portion 12 and removed.

It will be appreciated that it is also possible for an optical securityelement according to the invention to be part of a transfer, sticker orlaminating film or to be formed by a stamping film, a sticker film, atransfer film or a laminating film. In addition it will be appreciatedthat it is also possible for an optical security element according tothe invention, besides the layers 13, 14, 15 and 16 shown in FIG. 1, tohave further layers. Such further layers can be for example (coloured)decorative layers or layers of a thin film layer system which producescolour shifts in dependence on the angle of view, by means ofinterference.

Furthermore it is also possible for the reflection layer 15 to be onlypartially implemented or to be entirely dispensed with so that theoptical security element acts as a transparent and not as a reflectivesecurity element. It would also be possible to dispense with theadhesive layer 16.

The precise shape of the relief structure 17 and the optical effectsproduced by the relief structure 17 will now be described with referenceto FIGS. 2 a to 2 c:

FIG. 2 a shows a partial surface region 21 having a pattern region 23and a background region 22, as well as a part of a verification element20 with three line gratings 26. FIG. 2 b shows a surface region 27 witha background region 28 and two pattern regions 29 and 30, the partialsurface region 21 showing a part from the surface region 27.

As can be seen from FIGS. 2 a and 2 b a relief structure is shaped inthe surface region 27 and in the partial surface region 21 respectively,the azimuth angle of the relief structure being varied in a serpentineline form in dependence on the value of the X-axis.

The relief structure is preferably shaped in the replication layer 14 bymeans of an electron beam lithography system which permits periods inthe submicron range to the micron range or by means of aphotolithographic process which permits periods of less than 1 μm. Inthat respect the spatial frequency of the relief structure is about 1000lines per millimetre. The period of the parameter variation functionwhich varies the azimuth angle of the relief structure 17 periodicallybetween +40 degrees and −40 degrees is preferably 20 to 300 μm. Theparameter variation function is a sine function. It will be appreciatedthat it is also possible to use another periodic function as theparameter variation function or to provide other minimum/maximum azimuthangles.

In addition the views in FIGS. 2 a and 2 b serve only to explain thefunctional principle and are not true to scale. Usually the patternregions 23, 30 and 29 are of dimensions which correspond to a multipleof the period of the parameter variation function and at any event varyin a range which can be resolved by the human eye.

The partial region 23 is of a width which corresponds to the length of aperiod 25 of the parameter variation function and is thus for example100 μm wide. As can be seen from FIGS. 2 a and 2 b the azimuth angle ofthe relief structure 17 is varied in the background regions 22 and 28and the pattern regions 23, 30 and 29 by parameter variation functionswhich are phase-displaced relative to each other through 180 degrees andwhich are otherwise identical. As indicated in FIG. 2 d the parametervariation function used in the pattern region 23 is thus displaced byhalf a period length 24, that is to say by 50 μm, with respect to theparameter variation function used in the background region 22. A phasedisplacement of 180 degrees permits a particularly great contrastbetween the pattern region and the background region. It will beappreciated that it is also possible in that respect to deviate somewhatfrom the phase displacement through 180 degrees. In addition advantagescan also be enjoyed in deviating considerably from a phase displacementof 180 degrees and providing for example a phase displacement of 45degrees or 135 degrees, in the one pattern region or the other. Thus itis possible for example to implement concealed grey scale images inwhich the grey scale is encoded by means of the phase displacement.

Without use of the verification element 20 the surface region 27 nowappears to be homogeneous to the human viewer as the mean azimuth anglewhich can be resolved by the human eye is constant in the patternregions 29 and 30 and in the background region 28 surrounding them. Ahomogeneous optical effect which is dependent on the viewing angle isthen afforded for the viewer in the surface region 28, that effect beingdependent on the azimuth angle range covered by the parameter variationfunction and on the selected spatial frequency of the relief structure17.

FIG. 2 c now shows the situation in which the verification element 20 isplaced on the partial surface region 21. The light source is in the Y-Zplane so that the k-vector of the light does not have any Y-component.

FIG. 2 c shows the partial surface region 21, the line grating 26, thepattern region 23 and the background region 22. In addition FIG. 2 cshows an optical impression 31 of a viewer who is viewing the partialsurface 21 from the left-hand side and an optical impression 32 of aviewer who is viewing the partial surface region from the right-handside.

As shown in FIG. 2 c the grating lines 26 of the verification element 20cover only the surface regions of the background region 22 with negativeazimuth angles and only surface regions of the pattern region 21 withpositive azimuth angles. If the partial surface region 21 is viewed froma negative azimuth angle, that is to say from the left, the backgroundregion 22 is accordingly dark and the pattern region 23 light. If thepartial region 21 is viewed from the positive azimuth angle side, thatis to say from the right, the background region 22 is light and thepattern region 23 dark.

The optical impression 31 thus shows a coverage 312 by the grating line26, dark regions 311 and 314 in the region of the background region 22and a light region 313 in the region of the pattern region 23. As acounterpart the optical impression 23 exhibits coverage 322 by thegrating line 26 and light regions 321 and 324 in the region of thebackground region 22 and a dark region 323 in the region of the patternregion 23.

In real viewing the coverages 312 and 322 disappear as the period of theparameter variation function ranges in an order of magnitude which canno longer be resolved by the human eye. Accordingly, light patternregions and dark background regions are provided for the viewer from theleft-hand side and dark pattern regions and a light background regionfor the viewer from the right-hand side. If the verification element 20is displaced by half a period of the parameter variation function, thecontrary impression is afforded, that is to say a light backgroundregion and dark pattern regions when viewing from the left-hand side anda dark background region and light pattern regions when viewing from theright-hand side. If accordingly the optical security element is viewedthrough the verification element 20, dynamic control of the lightnesscontrast is afforded.

FIGS. 3 and 4 now show two further embodiments of the invention in whichthe azimuth angle of the relief structure 17 is varied by a periodicparameter variation function.

FIG. 3 shows a surface region 33 with a background region 34 and apattern region 35 and a partial region of the verification element 20with a plurality of grating lines 26.

The period of the parameter variation function shown in FIG. 3 is 50 μmso that the line spacing of the grating lines 26 is here also 50 μm. AsFIG. 3 shows the background region 34 is formed by six partial regions341 to 346. The partial regions 341 to 346 each involve the width of aperiod of the parameter variation function which is a function which iscomposed periodically of parabolic portions. The pattern region 35 isformed by two partial surfaces 351 and 352 which are also of the widthof a period of the parameter variation function.

As in the embodiment of FIGS. 2 a and 2 b, either the negative azimuthangle regions of the background region 34 and the positive azimuth angleregions of the pattern region 35 or the positive azimuth angle regionsof the background region 34 and the negative azimuth angle regions ofthe pattern region 35 are covered by the grating lines 26. That givesthe effect described with reference to FIG. 2 c, wherein the appearanceis somewhat different, from different viewing directions, in comparisonwith the surface region 27, by virtue of the differing parametervariation function.

FIG. 4 shows a surface region 4 which is composed of a plurality ofpartial regions 40 to 49. The surface regions 40 to 49 are each formedby respective identical diffraction structures which each have aplurality of concentric rings arranged in a circular configurationaround the centre of the respective partial surface. The width andheight of a partial surface is about 100 μm while the spatial frequencyof the diffraction structure is about 1000 lines per millimetre.

Accordingly FIG. 4 shows an example of a periodic parameter variationfunction in which the azimuth angle of the diffraction structure 17 isvaried periodically in dependence on the value of the X-axis and Y-axis.That function therefore exhibits a periodicity both in the X-axis andalso in the Y-axis so that items of concealed information can be readout with a differing orientation of the verification element 20. Patternregions are now placed in the surface region 4 in accordance with thekind shown in FIG. 3 so that the partial surfaces 41 to 46 are coveredby identical but phase-displaced partial surfaces. In this case a phasedisplacement of the partial surfaces of a possible pattern is possibleboth in the X-direction and also in the Y-direction, depending on therespective choice of that phase displacement the pattern region can thenbe read out when the grating is oriented in the Y-direction or in theX-direction respectively.

A further embodiment of the invention is described hereinafter withreferences to FIGS. 5 a to 5 d, in which the relief structure is adiffraction grating whose spatial frequency varies periodically inaccordance with the parameter variation function.

FIGS. 5 a to 5 c show three different parameter variation functions 53,54 and 55 which vary a spatial frequency 52 in dependence on a value 51of the X-axis of the surface region. The k-vector of the reliefstructures described with reference to FIGS. 5 a to 5 c is oriented inthe direction of the Y-axis so that the grooves of the relief structureare oriented parallel to the X-axis. The grating lines 58 are alsooriented parallel to the X-axis.

The parameter variation function 53 is a sawtooth-shaped function whichvaries the spatial frequency in the range of 800 lines per millimetre to1200 lines per millimetre in a sawtooth-shaped configuration. The periodof the parameter variation function is 50 μm. At the minima of theparameter variation function 53, that is to say at a value of 800 linesper millimetre, there is a red colour impression which then changeslinearly to a blue colour impression at the next maxima at 1200 linesper millimetre. Within a period accordingly the colour impressionchanges from red to blue. In that respect the colour impressions relateto a typical illumination/viewing angle combination.

The parameter variation function 54 is a triangular function with aperiod of 100 μm, which varies the spatial frequency of the diffractiongrating from a minimum value of 800 lines per millimetre to a maximumvalue of 1200 lines per millimetre and back. Accordingly within a periodthe colour impression changes from red to blue and back from blue to redagain.

The parameter variation function 55 is a sine function with a period of100 μm, which varies the spatial frequency of the diffraction grating independence on the value of the X-axis from a minimum value of 800 linesper millimetre to a maximum value of 1200 lines per millimetre and back.Accordingly within a period there is a colour impression from red toblue and back to red.

As the period of the parameter variation functions 53 to 55 lies belowthe resolution capacity of the human eye, the viewer, within the surfaceregion, has a unitary colour impression arising out of the mix of thecolour spectrum determined by the parameter variation function. If now averification element 57 with the grating lines 58 which have a linespacing 56 corresponding to the period of the parameter variationfunction is applied to that diffraction grating, then a given part ofthe colour spectrum is respectively covered by the grating lines 58 sothat the colour impression changes upon movement of the verificationelement over the diffraction grating.

FIG. 5 d now shows a surface region 50 with a background region 501 anda pattern region 502. In the background region 501 the spatial frequencyof the relief structure is varied in accordance with the parametervariation function 54. In the pattern region 502 the spatial frequencyof the relief structure is varied with a parameter variation function 54which is phase-displaced by half a period, that is to say by 50 μm. Ifnow the verification element 57 with the line grating spacing 56 ismoved over the surface region 50, respectively different colour regionsare covered in the background region 501 and the pattern region 502 sothat the pattern region 502 gives the human viewer a different colourimpression from the background region 501. If the verification element57 is thus moved over the pattern region 50, there is for examplefirstly the impression of a blue pattern region and a red backgroundregion which then, with the movement of the verification element,steadily goes to a red pattern region against a blue background region.

As already stated hereinbefore with reference to FIG. 2 b FIG. 5 dserves only to explain the operating principle involved. Usually patternregions are of an extent which embraces a plurality of periods of theparameter variation function and which is in an order of magnitude whichcan be resolved by the human eye.

Accordingly the parameter variation function on the one hand determinesthe homogeneous colour impression which is produced in the situation inwhich no verification element is applied to the surface region 50. Inaddition the parameter variation function determines how the colourchanges upon displacement of the verification element over the surfaceregion 50 (for example abrupt colour changes when using the parametervariation function 53), this serving as an additional security feature.

Reference will now be made to FIGS. 6 a and 6 b to describe a furtherembodiment of the invention in which the relief structure is adiffraction grating whose profile depth is varied periodically inaccordance with the parameter variation function.

FIG. 6 a shows a relief structure 61 whose profile depth is varied witha constant spatial frequency by a periodic parameter variation functionwith a period 63.

The relief structure 61 is preferably a first-order diffractionstructure (spatial frequency range varies in the range of thewavelength) or a zero-order diffraction structure (line spacing is lessthan the wavelength of the light). The profile depth is altered by theperiodic parameter variation function more slowly in comparison with thespatial frequency of the diffraction grating in dependence on the valueof the X-axis or in dependence on the value of the X-axis and theY-axis. The period of the parameter variation function is between 10 μmand 100 μm and is preferably of a value around 100 μm.

The relief structure 61 shown in FIG. 6 a thus involves for example aspatial frequency of 1000 lines per millimetre and the line spacing 62is thus 1 μm. The period 63 is 1000 μm and the profile depth is variedperiodically in dependence on the value of a Y-axis 69 between 0 nm andfor example 150 nm.

FIG. 6 b now shows a surface region 65 defined by an X-axis 68 ad theY-axis 69, with a background region 66 and a pattern region 67, in whichthe relief structure 61 is varied periodically in the direction of theY-axis, as shown in FIG. 6 a. As shown in FIG. 6 b, in the patternregion 67 the parameter variation function is displaced by half aperiod, that is to say by 50 μm, with respect to the parameter variationfunction of the background region 66.

If now the surface region 65 is viewed through a verification elementwith a line grating of 100 μm, then regions of different profile depthsare covered in the background region 66 and the pattern region 67 sothat the pattern region 67 no longer appears homogeneous. When theverification element is applied, there is thus a brightness contrastbetween the pattern region and the background region, which changes whenthe verification element is displaced. When thus the regions of thebackground region of around 150 nm profile depth are covered by the linegrating, the background region appears darker as the optical effectproduced in the background region is determined by profile depths around0 nm. Conversely the pattern region appears lighter in that position ofthe verification element. Upon displacement of the verification elementthat effect then slowly changes to the opposite so that the backgroundregion appears lighter and the pattern region appears darker.

A further embodiment of the invention will now be described withreference to FIGS. 7 a to 7 e, in which the relief shape of the reliefstructure is varied periodically in accordance with the parametervariation function.

FIG. 7 a shows a surface region 7 with a pattern region 74 and abackground region 73. The surface region 7 further has a periodicsuccession of partial regions 71 and 72 in the direction of the X-axis,wherein in partial regions 71 a relief shape 76 is shaped in thebackground region and a relief shape 75 is shaped in the pattern regionand in partial regions 72 the relief shape 75 is shaped in thebackground region and the relief shape 76 is shaped in the patternregion.

The width of the partial regions 71 and 72 is less than 300 μm so thatthe partial regions 71 and 72 cannot be resolved by the human eye. Therelief shapes 75 and 76 represent asymmetrical, mutually mirroredstructures so that the profile shape 76 can also be viewed as a reliefstructure 75 in which the azimuth angle is turned with respect to therelief shape 75 through 180 degrees. Typical spatial frequencies of theprofile shapes 75 and 76 are in the range of 1200 lines per millimetreto 150 lines per millimetre.

The width of the partial regions 71 and 72 is thus for example 50 μm ineach case so that the period of the parameter variation function of thesurface region 7 is 100 μm. The spatial frequency of the profile shapes75 and 76 is for example 1150 lines per millimetre.

If now the surface region 7 is viewed without using a verificationelement, a homogeneous impression is produced in the surface region 7for the human viewer, that impression corresponding to that of asinusoidal diffraction grating with a spatial frequency of the profileshapes 75 and 76, that is to say of 1150 lines per millimetre.

If a verification element with a period which corresponds or almostcorresponds to the period of the parameter variation function is appliedto the surface region 7 the pattern region 74 becomes visible. Dependingon whether the grating lines cover the profile shapes 75 or 76 of thepattern region 74, the viewer sees a dark pattern region against a lightbackground region or a light pattern region against a dark backgroundregion. If the surface region is turned through 180 degrees, the viewersees the complementary impression.

Thus for example when the partial regions 71 are covered by gratinglines 77 in the surface region 7 there is the effect shown in FIG. 7 d,that the pattern region 74 appears dark and the background region 73appears light. If the surface region 7 is turned through 180 degrees,that gives the situation shown in FIG. 7 e, of a light pattern region 74against a dark background region 73. As the period of the parametervariation function is less than the resolution capacity of the humaneye, the grating lines 77 are not visible to the viewer so that, whenviewing the surface region in the situation shown in FIG. 7d the darksurface region 74 against the light background region 73 is visible tothe viewer while in the situation shown in FIG. 7 e the light backgroundregion 74 is visible against the dark background region 73. Thatadditional tilting effect when viewing from directions which are turnedthrough 180° relative to each other forms an additional securityfeature.

It will be appreciated that it is also possible for the parametervariation function to be varied in the partial regions 71 and 72 betweenany other asymmetrical profile shapes. In addition it is also possiblefor the parameter variation function not to be a binary function whichdistinguishes between two different profile shapes, but for example forthe angle of inclination of the profile shape 75 to be varied linearlyin accordance with a sine function. That also affords additionalsecurity features which contribute to enhancing the level of safeguardagainst forgery.

In addition it is also generally possible to use parameter variationfunctions in which the relief shape changes periodically. The reliefprofile can thus be represented for example by a function:f ₃(x)=f ₁(x)+f ₂(x)in which:${f_{1}(x)} = {b\quad{\sin\left( \frac{2\pi\quad x}{d} \right)}}$${f_{2}(x)} = {\frac{b}{2}{\sin\left( {\frac{2\pi\quad x}{d/2} + {\beta(x)}} \right)}}$

Thus it is also possible for the parameter variation function to varythe relief shape of the relief structure periodically in accordance withthe parameter variation function, by varying the width of the troughs ofthe relief structure periodically in accordance with the parametervariation function.

That will now be illustrated by way of example with reference to FIGS. 8a to 8 e.

In a region 8 corresponding to a period of the parameter variationfunction the width of the troughs of the relief structure is reducedlinearly with a constant spatial frequency. As shown in FIG. 8 b in thatway a width 82 of the troughs of the relief structure is varied along anX-axis 81 in accordance with a function 83. The parameter variationfunction is for example a sawtooth-shaped function which varies thewidth of a rectangular relief structure with a line spacing of 300 nmbetween 230 and 70 nm. That gives a profile shape 84 (FIG. 8 c) in aregion a of the surface region 8, a profile shape 85 (FIG. 8 d) in aregion b of the surface region 8 and a profile shape 86 (FIG. 8 e) in aregion c of the surface region 8, wherein the profile shapes 84, 85 and86 have a line spacing 87 of 300 nm.

In accordance with the selected spatial frequency, optical effects ofdifferent colour and lightness are produced in the regions a, b and c sothat, with a phase displacement of the pattern and the backgroundregions, the optical superimposition effects already referred to abovewhen using a verification element/without using a verification elementare produced. The period of the parameter variation function is herepreferably also in the range of 40 to 300 μm.

In accordance with a further embodiment of the invention it is alsopossible to use a macrostructure of more than 300 lines per millimetreas the relief structure. Typical periods of such a macrostructure are 10μm. Macrostructures thus operate substantially by reflection and not bydiffraction. Two such macrostructures are shown in FIGS. 9 a and 9 b,wherein FIG. 9 a describes a macrostructure 91 with a period 93 and FIG.9 b describes a macrostructure 92 with the period 93. The period 93 isfor example 100 μm. If now the macrostructures 91 and 92 are viewedthrough a verification element with a line spacing corresponding to theperiod 93, different regions of the macrostructures 91 and 92 are thusvisible depending on the respective position of the verificationelement. In the pattern regions the macrostructure 91 is nowphase-displaced with respect to the macrostructure in the backgroundregion so that respectively different regions of the macrostructures 91and 92 are visible in the background region and in the pattern regionwhen using the verification element. If the verification element is notapplied the entire surface region appears homogeneous. When theverification element is applied there is a lightness contrast betweenthe pattern region and the background region.

The embodiments shown in FIGS. 1 to 9 d are each described withreference to the use of a linear line grating as an encodingsystem/verification element. As already mentioned hereinbefore howeverit is also possible, besides a linear line grating, to use further andin particular also two-dimensional gratings. FIG. 10 a thus shows alinear line grating 101 and FIGS. 10 b to 10 f show further linegratings 102 to 106 which can also be used in the embodiments shown inFIGS. 1 to 9 b.

In addition it is also possible to provide in a surface region patternregions which become visible with different gratings. Thus FIG. 11 showsa surface region 110 in which there are provided various pattern regions113 which respectively become visible at a given angle of inclination ofa verification element 11. If now the verification element 11 is rotatedon the surface region 112, that gives the effect of a moving image. Ifthe surface region 112 is viewed without a verification element 111, theimpression is that of a homogeneous surface region 110. Pattern regions113 of that kind can be implemented with the embodiments shown in FIGS.1 to 9 b, in which respect it will be appreciated that it is alsopossible for different embodiments to be combined together in order hereto achieve still further additional colour phenomena and securityfeatures which are dependent on the viewing angle.

In addition it is possible to use a verification element whose gratingis not identical to that of the parameter variation function. Thus theverification element can have for example a period which corresponds totwice the period or a multiple of the period of the parameter variationfunction. In addition the verification element can also shape out ahyperbolic pattern of a period corresponding to the period of theparameter variation function.

The above-described security features can be used as stand-alonesecurity features. It is however also possible for those securityfeatures to be combined with further security features within a securityproduct. Thus they can be parts of an OVD (optical variable device), forexample a Kinegram® or Truststseal® and thus for example form thebackground of a Kinegram®. It is also possible for the above-describedsecurity features to be arranged mosaic-like in an OVD.

1. An optical security element having a substrate layer, wherein arelief structure defined by relief parameters is shaped in a surfaceregion of the substrate layer, which region is defined by an X-axis anda Y-axis, for producing an optically perceptible effect, wherein one ormore of the relief parameters defining the relief structure in thesurface region are varied periodically in accordance with a periodicparameter variation function, wherein the surface region is divided intoone or more pattern regions and a background region and wherein one ormore of the relief parameters defining the relief structure reliefshape, relief depth, spatial frequency and azimuth angle in thebackground region and the one or more pattern regions are variedperiodically in accordance with a periodic parameter variation function,wherein the relief structure is a diffraction grating and the period ofthe parameter variation function is between 20 μm and 300 μm, and theone or more of the relief parameters defining the relief structure,relief shape, relief depth, spatial frequency and azimuth angle in theone or more pattern regions are varied in accordance with a parametervariation function which is phase-displaced with respect to theparameter variation function of the background region.
 2. An opticalsecurity element according to claim 1, wherein the phase displacement ofthe parameter variation function between the pattern region and thebackground region is about 180 degrees.
 3. An optical security elementaccording to claim 1, wherein the phase displacement of the parametervariation function between the pattern region and the background regionis selected in accordance with the contrast to be set.
 4. An opticalsecurity element according to claim 1, wherein the relief structure is adiffraction grating whose azimuth angle is varied periodically inaccordance with the parameter variation function.
 5. An optical securityelement according to claim 4, wherein the mean azimuth angle in relationto the resolution capacity of the human eye is constant.
 6. An opticalsecurity element according to claim 4, wherein the parameter variationvaries the azimuth angle of the diffraction grating periodically independence on the value of the X-axis.
 7. An optical security elementaccording to claim 6, wherein the parameter variation function variesthe azimuth angle of the diffraction grating in such a way that thediffraction grating is composed of a plurality of serpentine line-shapedlines.
 8. An optical security element according to claim 7, wherein theparameter variation function is a sine function which varies the azimuthangle of the diffraction grating in dependence on the value of theX-axis.
 9. An optical security element according to claim 4, wherein theparameter variation function varies the azimuth angle of the diffractiongrating periodically in dependence on the value of the X-axis and thevalue of the Y-axis.
 10. An optical security element according to claim9, wherein the parameter variation function varies the azimuth angle ofthe diffraction grating in such a way that the diffraction grating iscomposed of a plurality of lines arranged in concentric circles.
 11. Anoptical security element according to claim 4, wherein the diffractiongrating has a spatial frequency of more than 300 lines per mm, inparticular a spatial frequency of 800 to 1200 lines per mm.
 12. Anoptical security element according to claim 1, wherein the reliefstructure is a diffraction grating whose spatial frequency is variedperiodically in accordance with the parameter variation function.
 13. Anoptical security element according to claim 12, wherein the mean spatialfrequency in relation to the resolution capacity of the human eye isconstant.
 14. An optical security element according to claim 12, whereinthe parameter variation function varies the spatial frequencyperiodically between a maximum frequency, preferably 1200 lines per mm,and a minimum frequency, preferably 800 lines per mm, in dependence onthe value of the X-axis.
 15. An optical security element according toclaim 14, wherein the parameter variation function is a sawtoothfunction, a triangular function or a sine function.
 16. An opticalsecurity element according to claim 1, wherein the relief structure is adiffraction grating (61) whose profile depth is varied periodically inaccordance with the parameter variation function.
 17. An opticalsecurity element according to claim 16, wherein the parameter variationfunction varies the profile depth of the diffraction gratingperiodically between a maximum depth, preferably 300 nm, and a minimumdepth, preferably 50 nm, in dependence on the value of the X-axis. 18.An optical security element according to claim 16, wherein the parametervariation function is a triangular, rectangular or sine function.
 19. Anoptical security element according to claim 1, wherein the relief shapeis varied periodically in accordance with the parameter variationfunction.
 20. An optical security element according to claim 19, whereinthe relief shape is varied periodically between two asymmetrical,mutually mirror-symmetrical relief shapes.
 21. An optical securityelement according to claim 1, wherein the width of the troughs of therelief structure is varied periodically in accordance with the parametervariation function.
 22. An optical security element according to claim1, wherein the mean azimuth angle of the relief structure respectivelycorresponds to the azimuth angle of an associated verification grating.23. An optical security element according to claim 1, wherein the phasedisplacement between the background region and the pattern region isaccompanied by a further function change.
 24. A system for visualisingitems of concealed information comprising a security element having asubstrate layer in which a relief structure defined by relief parametersis shaped in a surface region of the substrate layer, which region isdefined by an X-axis and a Y-axis, for producing an opticallyperceptible effect, wherein one or more of the relief parametersdefining the relief structure in the surface region are variedperiodically in accordance with a periodic parameter variation function,wherein the surface region is divided into one or more pattern regionsand a background region, wherein one or more of the relief parametersdefining the relief structure relief shape, relief depth, spatialfrequency and azimuth angle in the background region and the one or morepattern regions are varied periodically in accordance with a periodicparameter variation function, wherein the relief structure is adiffraction grating and the period of the parameter variation functionis between 20 μm and 300 μm, wherein the one or more of the reliefparameters defining the relief structure relief shape, relief depth,spatial frequency and azimuth angle in the one or more pattern regionsare varied in accordance with a parameter variation function which isphase-displaced with respect to the parameter variation function of thebackground region, and wherein the system further has a verificationelement which has a verification grating which is defined by a periodictransmission function and whose period corresponds to the period of theparameter variation function.
 25. A system according to claim 24,wherein transmission function is a non-binary transmission function, inparticular a sine function.